The ability of cells to recognize one another and to engage in selective mutual contacts is fundamental to processes ranging from interactions of mating-types in microorganisms and of gametes infertilization, to morphogenetic cell assortment and multicellular organization in the vertebrate embryo. Cell recognition depends on the expression on the cell surface of molecular entities that function as affinity-determinants and as selective cell-cell ligands. The work proposed here focusses on recognition properties of embryonic (chick) neural retinal cells and their two-level expression: tissue-specific (determines selective contacts between various cell types and their association into the tissue pattern). Starting with the evidence that the previously demonstrated retina "cognin" is a determinant of tissue-specific recognition of retina cells, the present work will, first, answer the following questions: When does this cognin begin to appear in the eye region of the early embryo? It is involved also in formation of contacts between retina (optic nerve) and optic tectum? Second: The nature of type specific recognition of retinal cells will be investigated by detailed biochemical-immunological analysis of cell membrane proteins from the two main cell types, glia and neurons, to identify recognition-determinants specific for these cells. Third, having previously shown that BrdU inhibits development of cell type-specific (but not tissue-specific) cell recognition in the early embryonic retina, investigation of this experimental paradigm (by biochemical and immunological analyses) will seek to further identify the distinction between these two levels of cell recognition. Fourth, separation of glia cells from retina neurons triggers changes in contanct-affinities of glia cells and leads to their transformation into lens-type cells. We will investigate changes in membrane proteins in transforming glia cells to determine if there is loss of retinal recognition determinants and appearance of lens-specific determinants. Taken as a whole, these studies will advance the analysis of morphogenetic cell recognition from the level of tissue-specificity to the sofar unexplored, but fundamental issue of cell type-specificity.